Tractor has a test called “MicCompareA01” that makes it easy to compare the difference between two mics, including phase, in a production setting. For QA401 release 1.903 and Tractor release 0.994, some changes are coming that will be documented here. Note that this post is preliminary: We’re having some new fixturing made that will allow more meaningful conclusions to be drawn, and from that comparison a blog post will be prepared. But for now, please take this brief posting as the updated documentation on the feature.
First, ensure that the QAAnalyzer application (version 1.903 or later) is installed from Github.
Next, ensure that the Tractor application (version 0.994 or later) is installed from Github.
Run the QAAnalyzer application and ensure you can connect to the hardware and collect traces. When you first run the application and connect the hardware, you should see status change in the lower left of the application, finishing with “Connected”. Then connect the L+ output to the L+ input, and short the L- input with a BNC shorting block (cables and shorting blocks are not included). Press the Run button and the Gen 1 button to enable the generator. At this point, you’ve captured a tone in loopback mode. You can go into more detailed intro tests via the “Getting Started” guide located here.
To compare two microphones, we first need a hardware setup. For this test we’ll use the following pieces. The QA470 (top two boxes) are mic pre-amps with phantom power.
The QA401 Audio Analyzer sits below the mic-preamps.
On the very bottom is a QA460 transducer driver. This can drive a few watts of power into an 8 ohm speaker.
Not shown above is the acoustic setup. We’ll go over that in more detail once the fixturing is complete. For now, the mics are arranged as follows. These are two Earthworks M23R reference mics. Both mics are hand-tuned by the manufacturer and should be +/- 0.5 dB from each other. The mics are located in the far field of a sort-of full range speaker.
With the hardware setup above and the QA401 set to “run” we should be able to see left mic on the display. The right mic can be added by pressing the “Right” button the front panel. At this point, you should see any room noise causing a change to the display.
Stop the QA401 application.
Next, start the Tractor application. You should see the following screen:
Click the button “Add new test” and you’ll see
Select the “Other” test category and pick the test to add entitled “MicCompareA01”
Press OK and you’ll be taken back to the home screen:
In the right panel, there are some parameters to be configured. You can scroll that panel and you’ll see an “OK” or “Cancel” button at the bottom. Leave all the defaults the same, except for a few.
Change FFT to be 32. This specifies a 32K FFT
Change Display Y Min to be -10. Here we are specifying the window that will be shown on the analyzer after the test is complete.
Change Windowing to be 3 mS (unless you are working in a acoustic large work area, then you can go larger). This means that all room clutter, including the floor, ceiling, walls, furniture is at least ~3 feet from the mic and speaker. This also means that low-frequency data below about 300 to 400 Hz is suspect.
At the very bottom, click the “Check Phase” checkbox. This will ensure that phase is confirmed for the DUT mic and that the XLR connector isn’t wired backwards.
Notice the empty space for Mask File Name. We need to create a simple mask file. Click the Browse button:
Clicking that button will take you to the UserMasks directory, which is commonly located at:
c:/users/<USERNAME>/QuantAsylum/QA401/UserMasks
In that directory, create a file with a meaningful file name (eg “FirstMask.mask”). And inside that file, add the following lines:
# QuantAsylum Mask File
# Frequency, Minimum Pass Value (dB), Maximum Pass Value (dB)
300, -1.2, 1.2
5000, -1.5, 1.5
10000, -2, 2
This mask file will likely need to be changed depending on the mics you are using. What the file specifies is a range of frequencies and the min/max values required for the DUT mic to pass. Values are interpolated among the ranges. And if a frequency range isn’t included, then it’s not part of the mask. That is, the mask above doesn’t care what the result is at 200 Hz. But at 301 Hz, the measured value of the DUT (right channel) needs to be within 1.2 dB of the reference mic.
With that file created, specify that as the target in Test Details
.
Click on the OK button, and then go to the File menu and save your project. Now click the big “Run Test” button on the right. That opens the Test Task window. This is the screen that the operator will interact with.
Press the Start button. You should hear 1 or 2 chirps from the speaker, and a pass/fail message:
The QAAnalyzer screen appears as follows:
The blue trace shown is the difference trace between the left (reference) and right (DUT) microphone. We can also see the mask we created overlaid on the display, as well as a large PASS on the QAAnalyzer screen. Had the value of the Reference minus DUT mic wandered outside of the mask, then it would have been considered a fail.
And since we also checked the “Check Phase” box in the test details, we can also be sure the mics have matching phase.
As indicated above, the above isn’t being done in a good acoustic environment and as a result there’s a lot of peaking shown that shouldn’t be there. In the next week with new fixturing we’ll refine the setup and confirm the peaking is reduced and also take the comparison down to a lower frequency.
Summary
This short post showed how to quickly setup an automated test for comparing a reference mic with a DUT mic.